Abstract: We report on the first experimental determination of the Fermi /3 strength in the /3-decay of 8B, enabled by significant improvements in production yields at ISOLDE/CERN. This allows the direct observation of isospin mixing in the 16.6 and 16.9 MeV 2+ doublet in 8Be. Specifically, the 16.9 MeV state is resolved in /3-decay for the first time with sufficient statistics to test a widely used two-level model. With the high statistics we could perform two independent analyses: one based on the observed /3-strength distribution within the R-matrix framework, and a second based on the extracted /3-recoil broadening. The latter makes it possible to extract the Fermi and Gamow-Teller strengths up to the electron-capture region. The obtained strengths are fully consistent with the two-state isospin mixing model.